Many ascent and descent devices are known, some of which us a counterweight such as U.S. Pat. Nos. 4,458,781, 4,997,064, 6,161,639, 6,684,562, 7,198,134, German Patent DE 20216895, and French Patent FR 2440906. These citations may be characterized as having at least one of several attributes selected among counterweight, motorized, drum winder, sheave traction device, single or dual sheaves, and endless loop. While counterweight devices can maintain a constant assist load, a climber often has to adjust such assist force by manually selecting a physical counterweight. These devices represent assist methods for ladder climbing such as may be found in cranes, oil derricks, buildings, etc.
Patent DE 20216895 discloses an endless loop motorized, assist device with removable motor and load limiting using a slipping clutch device. In general, this type of system is limited to maintaining a constant speed up to a specific load level.
A more recent publication in WO 2005088063 discloses a motorized, endless loop, system using a variable frequency drive to the traction sheave and includes motion detection with load limiting and control. While this system attempts to keep tension at a constant level, it does not provide dynamic adjustment of the rate of assist to a climber.
Additionally, control mechanisms of related ascent and descent devices typically control stop and run climbing actions by providing a sensor in a control unit near the bottom of the system. For example, Tractel discloses a system that can start or stop the device by causing the lower sheave to rotate and displace a switch to start the motor. Other system, such as Avanti, employs a control algorithm based on timed events.